China faces a new HIV/STI challenge: HIV/STIs are spreading among the older adult population (e 50 years). The HIV/STI epidemics in older adults are driven by older female sex workers (e35 years), with the majority of HIV infected older adults reporting a history of cheap commercial sex with these women. Therefore, effective interventions must target older female sex workers - the main source of HIV/STI transmission. However, few epidemiologic data from community-based studies are currently available to examine the nature and contexts of sexual risk for HIV/STIs among older female sex workers in China. Our long-term goal is to develop an HIV/STI intervention that is culturally sensitive for older female sex workers. The objective in this particular application is to quantify the collective roles of social network components (relations, structures, and functions) and environmental factors (physical, social and political factors) on HIV/STI risk behavior in older female sex workers. The central hypothesis is that network-based interpersonal factors mediate the effects of environmental factors on HIV/STI risk behavior in female sex workers aged 35 years or older. The formulation of this hypothesis is based on the social ecological model, the feature of Chinese collectivistic culture, and our previous studies of social networks. We will achieve the objective by pursuing the following specific aims: (1) qualitatively describe environmental and social network factors for sexual risk among older Chinese female sex workers; (2) determine the prevalence of HIV and syphilis; and (3) quantitatively examine the collective roles of social network components and environmental factors on sexual risk for HIV/STIs. Aim 1 will be addressed by conducting in-depth interviews with older female sex workers and focus groups with stakeholders (e.g., pimps and owners of beauty salons and hotels). Aims 2 and 3 will be accomplished through a community-based survey of older female sex workers recruited from their social network ties in three cities in China (N=1,200; 400/site). This study is significant since (1) little is known about the environmental and social network factors for sexual risk among older female sex workers; and (2) findings will be used to develop the first theory-based HIV intervention that is culturally sensitive for older Chinese female sex workers in China. The application is innovative as (1) it is guided by the social ecological model and social network model; (2) it applies a novel network-based sampling method (i.e., respondent-driven sampling) to recruit more representative samples at the community level; and (3) it uses on- site bio-tests for HIV and syphilis and an on-site bio-test for prostate-specific antigen to validate self-reported condom use. The expected findings of the study will have an important positive impact, because, once the collective roles of the multi-faceted factors for sexual risk are known, intervention strategies can be developed to effectively target these factors, resulting in innovative approaches to the prevention of sexual risk for older female sex workers, which will ultimately reduce the sexual transmission of HIV to the older adult population.